In fiber optic systems, such as fiber-optic gyro (FOG) and coherent optical communication systems, the system's accuracy requires a precise knowledge of the optical signal phase. As an example, the rotation rate in a FOG system based on the Sagnac principle is determined by comparing the optical phases of two optical signals, e.g. light beams, propagating in opposite directions through an interferometric loop. Similarly, in a coherent communication system the information is encoded onto an optical signal by temporally varying the light beam's phase in the transmitter, and the encoded information is decoded at the receiver by comparing the phase of the transmitted light beam to the phase of a reference light beam.
To obtain optimum performance in these phase sensitive systems it is critical that the optical signal in the fiber is linearly polarized, and confined to one of the orthogonal polarization modes. The two orthogonal polarization modes (TM and TE) are not degenerate, i.e. they have slightly different phase velocities. If power is coupled from one polarization mode to the other the optical phase at the FOG detector (or the communication system receiver) will be perturbed. This results in drift errors for the FOG system and increased noise and signal fading in the coherent optical communication system. It is obvious, therefore, that phase sensitive optical systems must be fabricated using optical components which are polarization preserving.
One standard optical component required in each system is a polarization preserving, N.times.M star coupler. These N input / M output star couplers may range from, and include, 1.times.M splitters to N.times.1 multiplexers. The star coupler accepts optical signal power from N inputs, combines the N input powers into a single guide, and then splits the guide output into M equal parts.
The key parameters for the coupler are: (i) the splitting uniformity, (ii) the insertion loss (sum of the M output powers divided by the sum of the N input powers), and (iii) the polarization extinction (the output power in the desired polarization mode divided by the output power in the undesired polarization mode). The ideal coupler would have a uniform splitting ratio, zero excess insertion loss, and infinite polarization extinction.
Some of the prior art polarization preserving star couplers are fiber optic devices, which are fabricated either through the fusion elongation method or the mechanical polishing method. In the fusion elongation method 2.times.2 (N.times.N) star couplers are made by thermally fusing two polarization preserving optic fibers, and then elongating them in the waveguide portion. The cores of the two fibers must be in close proximity when fused so that in operation power transfer can occur between fibers via evanescent coupling.
The polarization axes of the fused fibers must also be perfectly aligned in parallel to preserve polarization modes in this fused coupling region. In the mechanical polishing method, the polished surface of two optic fibers are joined using an index matching liquid bond. Power transfer again occurs through evanescent coupling, so that the fibers must be in close proximity and must be aligned.
Both fabrication methods produce devices which have low excess insertion loss and relatively uniform splitting. However, the polarization extinction ratio is typically degraded due to angular misalignment of the polarization axes of the two fibers, unless the fiber geometry is designed to physically establish the main polarization axis easily, such as with the use of rectangular fibers. For these reasons, fiber star coupler configurations larger than 2.times.2 are impractical. One alternative is to cascade a series of 2.times.2 couplers to achieve the desired N.times.M result. Another alternative is to use feedback looping and tapping.
The prior art IO star couplers are fabricated by cascade arrangement of 1.times.2 splitters in tree structures. The splitters may be Y-junctions or directional couplers, and are fabricated on a substrate material. The substrate materials include glass and LiNbO.sub.3. For glass substrates the splitter circuitry is deposited on the substrate surface using an ion exchange method. In the case of LiNbO.sub.3 substrates, the titanium diffusion method is used to deposit the circuit configuration.
The prior art IO star couplers are polarization maintaining to a degree; similar to the polarization preserving characteristics of the optic fiber couplers. They are not single polarization devices, i.e. they do not have high polarization extinction.